Conventionally, there is known a casting apparatus for manufacturing die cast products by injecting a molten metal supplied into a tubular sleeve with use of a plunger at a high speed while pressurizing the molten metal, and by pressing the molten metal into a die (cavity) constituted of a fixed die member and a movable die member through a narrow gate. In recent years, vehicle components such as a cylinder block of an engine made of aluminum alloy are manufactured, using the aforementioned casting apparatus.
In the aforementioned casting apparatus, if air is stagnated in the tubular sleeve, the air may be drawn into the die while being trapped in the molten metal. As a result, product defects called blowholes (cast blowholes) may occur. Also, the air in the die may be trapped in the pressurized molten metal, and blowholes may be formed. In view of the above, in the conventional casting apparatus as described above, as disclosed in Patent Literature 1, for instance, air is sucked from the interior of the tubular sleeve through a gap between the tubular sleeve and the plunger, and additionally, air is sucked from the interior of the die for preventing blowholes as described above.
The casting apparatus described in Patent Literature 1 is advantageous in preventing product defects such as blowholes. However, as the degree of vacuum of the interior of the cavity is higher than the degree of vacuum of the interior of the tubular sleeve, a phenomenon called “top molten metal”, which may cause product defects (cold shuts), may occur. A top molten metal is a molten metal that is drawn from the interior of the tubular sleeve into the cavity prior to injection.
As a method for preventing a top molten metal, conventionally, air is sucked from the interior of the tubular sleeve and from the interior of the cavity so that the degree of vacuum of the interior of the cavity is lowered than the degree of vacuum of the interior of the tubular sleeve. However, even when the aforementioned operation is performed, if wear of the tubular sleeve or of the plunger progresses, the gap between the tubular sleeve and the plunger increases, which may lead to a negative pressure leakage. This obstructs an increase of the degree of vacuum of the interior of the tubular sleeve, and may cause formation of a top molten metal. In view of the above, in the actual practice, air is sucked from the interior of the cavity so that the degree of vacuum of the interior of the cavity is sufficiently lower than the degree of vacuum of the interior of the tubular sleeve, assuming that wear of the tubular sleeve or the like may progress, for preventing a top molten metal.
In other words, air is sucked from the interior of the tubular sleeve or from the interior of the cavity in order to prevent blowholes. In order to accomplish the above object, it is desirable to increase the degree of vacuum of the interior of the tubular sleeve or of the interior of the cavity. In this case, however, a top molten metal is likely to be formed. On the other hand, when the degree of vacuum of the interior of the tubular sleeve or of the interior of the cavity (in particular, the degree of vacuum of the interior of the cavity) is lowered, formation of blowholes is not sufficiently prevented, although formation of a top molten metal is prevented. Thus, there is a trade-off between preventing a top molten metal and preventing blowholes. In view of the above, it is desirable to solve the aforementioned trade-off problem in order to enhance productivity of cast products, while increasing the yield.